The invention relates to a method of controlling the engine speed of an electric retractor motor of a belt retractor in an especially load-independent manner.
From the state of the art various methods are known to eliminate a belt slack of a vehicle seat belt. It is the objective of said methods to safeguard prior to a case of restraint that the webbing of the seat belt contacts the vehicle occupant so that during deceleration of the vehicle no unnecessary forward movement of the vehicle occupant takes place and thus the latter can participate m the total deceleration of the vehicle as quickly as possible.
On the one hand, retraction of the webbing and elimination of the belt slack is intended to be performed very quickly in the so called pre-crash mode, i.e. when a critical driving situation is given. On the other hand, the belt slack is desired to be eliminated as conveniently as possible for the vehicle occupant during normal driving operation and, resp., after fastening the seat belt (in the comfort mode). From EP 1 661 478 B1 a method is known, for example, which as a command variable for controlling said process makes use of the motor torque of an electric retractor motor that is resulting from the motor current. When the webbing is in contact with the vehicle occupant, a higher torque is required to wind up the webbing so that the motor current increases. The increase in the motor current can be measured and, upon reaching a defined value, the motor torque can be reduced. In this method it is a problem that with a large belt slack at first no or only a low resistance is provided when the webbing is retracted. In this case, the webbing can be retracted very quickly, which may result in irritations of the vehicle occupant.
In order to eliminate this problem methods are known for measuring the speed of the belt reel and, resp., the retracting speed of the webbing via additional sensors, for example Hall sensors. These systems enable an adaptation of the motor speed and thus of the retracting speed of the webbing to values comfortable to the vehicle occupant.
These methods require additional sensors, however, which render the belt retractor more complicated and thus more expensive. In addition, for these sensors and a possibly required evaluating and control unit additional construction space is necessary.